Disk drive data storage subsystems have evolved from very large peripheral devices having large rotating data storage disks and hydraulic head positioning mechanisms into disk drives having very small, fully enclosed packages for inclusion entirely within the housing of small computing systems, such as personal computers and work stations. Concomitantly, non-removable storage disk diameters have progressed inwardly from e.g. 24 inches to as small as two and one half inches, with corresponding dimensional reductions in overall disk drive packages.
Overall external package height, width and length dimensions of a disk drive package have come to be referred to in the disk drive art by a single expression, "form factor". The term "form factor" means the external dimensional outline required for the disk drive subsystem including its on-board control electronics. While disk drive form factors including disk diameters have progressively shrunk, e.g. from 14 inch, to eight inch, to five and one quarter inch, to three and one half inch, to two and one half inch disk diameter based packages, the aerial data storage density at the data surface of the storage disk has increased dramatically as has the large scale integration of control electronics. Physically very small disk drives of the present time thus provide the same or greater user data storage capacity than characterized much larger disk drives of a few years ago.
Contemporary disk drives typically include at least one rotating data storage disk and a head positioner for positioning a data transducer head among multiple concentric data storage tracks on the data surface of the disk. Data storage capacities are conventionally increased by providing ganged multiple heads and commonly journalled multiple disks rotating about a central axis. Increased data track densities have been achieved by head positioning servo techniques which have promoted head alignment with the data storage track irrespective of dimensional changes brought on by thermal shifts within the disk drive package.
In some prior disk drives, a dedicated servo surface has been provided with factory recorded concentric servo tracks which are followed by a servo head. While servo surface technology has enabled significant increase in the number of data track locations (cylinders), the drawback has been the high cost of dedicating an entire disk surface to servo information and providing a separate servo read head and channel to the head position servo function.
Another approach pioneered by the assignee of the present invention has been to have a head positioner servo loop with an optical position transducer tightly coupled to a rotary head positioner actuator structure and also provide thermal correction information on one or more data surfaces of the disk. One example of a disk drive in accordance with this approach is to be found in commonly assigned U.S. Pat. No. 4,639,798. Another example of this type of disk drive architecture is provided by commonly owned U.S. patent application Ser. No. 07/192,353, filed on May 10, 1988, now U.S. Pat. No. 5,005,089.
Another prior approach to head positioning has been to embed servo information within each physical block storage location or "sector" of the data storage format on each data storage surface. One example of embedded sector servo information within a disk drive is described in commonly owned U.S. Pat. No. 4,669,004, the disclosure of which is hereby incorporated by reference. Data sectors carrying embedded servo information provide a number of advantages over other positioner systems. These advantages flow basically from the fact that the data head reading the data of interest also reads head position correction information and passes that information through the same read channel as the data. Thus, there will not be positional offsets or discrepancies as may occur between a data surface and data head, and a servo surface and servo head.
While embedded servo information within data block sectors overcomes the drawback of the dedicated servo surface approach, and may provide for increased track densities over the optical positioner architecture noted above, one drawback of sectorized, embedded servo information is that of susceptibility of the sampled data servo loop to mechanical vibrations. The embedded sector based servo loop typically samples the embedded servo information at a sampling rate which may be or become very sensitive to interferences from mechanical vibrations within the head positioning structure. Care must be given in the design of the disk drive to head positioner resonances in order to avoid interference with the operation of the sampled data servo loop.
As taught by the Banck U.S. Pat. No. 4,398,228, for example, it is theoretically desireable to tune or adjust head arm resonance to the servo sample rate which has an inherent notch in transfer function. The notch at the sampling rate thereby cancels the disturbance of the head arm resonance and provides a stablized servo loop response. There are several practical drawbacks to the Banck approach. First, the Banck disclosure did not provide any practical suggestions as to how to tune a head arm positioner structure of a fixed disk drive in order to make its resonance(s) coincide with the sampling rate. Moreover, the Banck disclosure did not recognize that with a fixed disk head positioner structure, there may be more than one head positioner resonance and mode (including torsional, bending, and lateral modes of vibration), and that the modes and resonance frequencies thereof occur at different frequencies. Also, as noted by Banck, head arm resonance at or near the zero dB crossing of the servo loop bandwidth, or a resonant frequency which is an integral multiple of one half of the sampling frequency above the servo bandwidth zero dB crossing, can lead to servo loop instabilities through "aliasing", i.e, high frequency resonance appearing in the base-band frequency range which is from zero to one half the sampling rate.
Thus, with embedded sector servo control systems for head positioning, a hitherto unsolved need has arisen for a mechanism for not only strengthening the positioner structure for the head, but also providing a number of selectable configurations so that interfering resonances, whether within the servo loop bandwidth or aliases, may be adjusted to minimize disturbance of the positioner system.
As disk drives have matured, disk drive form factors have tended to become standardized within the computer industry, so that makers of computers may obtain disk drives from a variety of sources which will fit within a well or space in the computer designed to accomodate a fixed disk drive of a given form factor. This way, data storage devices including disk drives, and perhaps tape backup drives for example, having a range of features, capacities and performance characteristics may be installed and used within a given family of computers.
With a standardized form factor as a controlling constraint in designing new disk drives, a hitherto unsolved problem has arisen in designing miniature disk drives having more than just a few disks. In such miniature disk drives, limited angular displacement, direct drive rotary actuators have been proposed and widely used, see commonly assigned U.S. Pat. No. 4,783,705, and the above-referenced U.S. Pat. No. 4,669,004, for example, which illustrate one practical form of in-line actuator structure. Such actuators which include ganged, comb-like extensions for connecting and supporting the head arms, flexures and heads, have been realized with lower mass and with higher track seeking movement velocities which achieve improved track access characteristics.
With linear actuator structures, it has been possible to conserve headroom between disks by laterally offsetting adjacently opposed heads and supporting structures. However, with commonly ganged rotary head mounting structures, the heads must remain in vertical registration to maximize capacity by allowing a larger data band and minimizing the space needed for a head landing zone for parking the heads on the data surface in the absence of the air bearing provided by disk rotation. With a rotary voice coil actuator structure heretofore there simply hasn't been enough room to fit many disks closely together on a single spindle for rotation about a common axis and concomitantly provide sufficient stiffness within the head positioner structure for effective head positioning. This limitation has arisen on account of the spacing or distance required between the disks in order to accomodate the vertically aligned heads and head mounting structures of the ganged head positioner assembly. Heretofore, a minimum spacing was required between each oppositely facing data surface of two adjacent disks in order to have enough room for the two heads (sometimes called sliders) and the suspension, load beam and head arm associated with each head.
Another and related limitation concerns the limited amount of torque available at the spindle to rotate the heads. As disk drives progressed to the five and one quarter inch diameter form factor in both full height and half height configurations, standard practice evolved to include a direct drive spindle motor as a part of the disk spindle assembly. As disk drive form factors have progressed to the three and one half inch diameter and the two and one half inch diameter form factors, where three or more disks have been used, the spindle motor has typically been included entirely within the spindle hub at the central region of the disk stack. Generally speaking, the torque generated by a direct drive, brushless spindle motor has been a function of the radius to the rotating magnet assembly. With the rotating magnets included within the radial constraints of the spindle hub, these "in the hub" spindle motors generally develop less torque than other spindle motor designs not constricted by the hub cross dimension.
Prior fixed disk drives having non-removable media have typically provided lubricated or carbon-overcoated disk data storage surfaces, so that the head slider could take off and land on the disk surface as the disk began to rotate on power-up, and ceased to rotate on power-down. With the slider in contact with the data surface, starting friction or "stiction" has required much greater spindle torque than the torque required merely to rotate the disks once the head sliders are "flying" on an air bearing. With the addition of heads and disk surfaces to increase data storage capacity, the stiction torque requirements have progressively increased, while the "in the hub" motor size has stayed constant or decreased, in order to maintain the form factor required by the computer industry.
Stiction within a disk drive having direct head-disk surface contact is a function of several characteristics including contamination within the enclosed space of the head-disk assembly and surface condition of the disk (and slider). In essence, a disk drive having direct head-disk contact is a very sensitive sensor for the presence of contaminants, whether due to out-gassing of components, or leakages of lubricants from the disk spindle or the actuator assembly, etc. No mass produced disk drive is perfectly free of contaminants, and resultant stiction must be overcome with sufficient torque from the spindle motor in order to achieve reliable, repeatable performance. The surface condition of the data storage media has also played an important function in stiction level. With coated oxide media, the level or amount of lubrication applied to the storage surface to protect against landing by the slider has directly affected stiction. In more recent thin film media applied by sputtering, the amount of molecular overcoat applied to harden and protect the thin film deposition also has affected stiction. If the disk surface is made too flat, the slider will stick to the disk surface by virtue of the relative vacuum formed between the oppositely facing surfaces in contact. This is particularly a problem with data storage disks formed of glass substrate material.
Nominally, the coefficient of friction (mu) will be less than 0.5. For example, if a load force of 9.5 grams exists between the head slider and the disk surface, with a mu of 0.5, half the load force will be required to cause relative sliding motion between the slider and the data storage surface when they are in oppositely facing, sliding contact. When the slider sticks to the data storage surface (stiction), the mu will often increase to 1.0. This increased force must be overcome by starting torque generated by the spindle motor before the disk drive may function as intended.
In order to overcome the additional torque requirements imposed by stiction, dynamic head loading techniques have been proposed for loading the heads into flying orientation only after the disks have reached operating angular velocity and for unloading the heads prior to power-down. Dynamic head loading mechanisms have required additional space between the disks and have not proven to be very stable.
As briefly mentioned above, a third and still related limitation of prior art head mounting structures has been the limitation imposed upon servo bandwidths and head settling times on account of vibrational modes of the head arm, flexure, and head. At least one or more of these vibrational modes, referred to hereinafter as first bending mode, first torsional mode, lateral mode, second bending mode and second torsional mode, has contributed to relative head motion of a servo head leading to uncorrectable servo error phase shifts at a frequency which effectively marks the bandwidth limitation of any servo loop including a servo head moved by the ganged head actuator assembly. While addition of damping material has attenuated somewhat the undesireable servo loop phase shifts associated with the first and second torsional modes, addition of damping structures adds time and costs to the actuator structure. Thus, a need has arisen for an selectable array of combinations of head suspension arrangements which may be selected and used for increasing the vibrational frequencies, and avoiding aliasing, otherwise limiting servo bandwidth, so that data tracks may be placed more closely together and so that settling times following head seeking to destination track location are reduced.
Yet another limitation associated with some disk drives of the prior art has been the provision of dynamic head loading and unloading mechanisms which merely lift the data transducer heads off of the data surface while permitting them to dangle in close proximity above the data storage surface in the unloaded position. Since the heads are gimbal mounted by flexures and in that sense behave as pendulums, severe shocks attributable to rough handling have caused the unloaded heads to crash into the data storage disks and thereby damage or destroy them.
One other limitation associated with other disk drives of the prior art has been the provision of dynamic head loading and unloading mechanisms which engage head mounting load beams in a middle region, thereby causing the head end region to cantelever above the disk surface and requiring more vertical headroom for unloading due to amplification of head loading mechanism tolerances.
Thus, a hitherto unsolved need has arisen for an improved miniature fixed disk drive having a significantly higher data storage capacity by disks which are spaced closer together and which are rotated by an in-the-hub direct drive spindle motor, wherein the rotary head mounting structure manifests improved vibration characteristics leading to increased open loop servo bandwidths and reduced head settling times and wherein a dynamic head loading and unloading mechanism provides more positive control of head position during loading and unloading with less vertical headroom being required.